Iowa Farmers Surveyed On Corn Rootworm Resistance

Every year Iowa State University Extension provides private pesticide applicator training covering a variety of pesticide safety and pest management issues. In response to reported incidences of Bt-resistant corn rootworm, a series of remote "clicker" questions were developed to survey current corn rootworm management practices and assess the degree to which farmers perceived resistance in 2012.

RESISTANT ROOTWORMS: When asked if they had fields in 2012 where they suspected Bt trait failure, almost one-fourth (23%) of farmers responded "yes." And 43% reported the suspected failure to someone. The majority of respondents who suspected trait failure either dug and rated root injury (38%) or noted goosenecking/lodging symptoms (33%) to assess the injury. About 13% of people who suspected a Bt trait failure were able to verify the larval injury.

The questions were asked at 153 pesticide applicator training sessions facilitated by ISU Extension field agronomists. More than 8,100 meeting participants responded to the six multiple-choice questions. Responses were anonymous and not everyone answered every question. Following are the statewide summaries of selected questions from the survey based on percentages of respondents. Erin Hodgson, ISU Extension entomologist; Aaron Gassmann, ISU research entomologist; and Kristine Schaefer, manager of ISU's pest management and education program, provide this information and observations.

To ascertain current management practices, participants were asked how they manage corn rootworm (see Figure 1). Of the various management options for corn rootworm, 17% of respondents used Bt traits, 25% used crop rotation and 38% responded they used three or more methods.

Figure 1: How do you mange corn rootworm?

When asked if they had fields in 2012 where they suspected Bt trait failure, almost one-fourth (23%) of participants responded "yes" and 43% responded that they reported the suspected failure to someone. The majority of respondents who suspected trait failure either dug and rated root injury (38%) or noted goosenecking/lodging symptoms (33%) to assess the injury (see Figure 2). About 13% of people who suspected a Bt trait failure were able to verify the larval injury (see Figure 3).~~~PAGE_BREAK_HERE~~~

Although lodging can occur as a result of rootworm feeding injury, there may be other causes such as strong wind events. The only way to verify Bt trait performance issues is to evaluate root injury. Thus, the rate of verified trait failures from this survey may be overestimated because larval root injury was not confirmed for all cases. Additionally, the level of root injury that a grower may have considered sufficiently high to warrant a trait failure is unknown. Generally, a trait failure would be considered to be present if there is one node of root injury for Bt corn with a single trait targeting rootworm and 0.5 nodes for corn that has two Bt traits targeting rootworm. A root rating scale developed by the Iowa State University Department of Entomology is available.

Figure 2: What method did you use to assess corn rootworm injury?

Figure 3: What was the outcome of the assessment?

To understand where most of the perceived Bt trait failures were occurring within Iowa, the ISU researchers grouped responses by region. About one-third of people in northeast Iowa suspected a Bt trait failure in 2012 (see Figure 4). Most of the positive verifications were in north-central Iowa, with approximately one-third of people reporting Bt trait failures (see Figure 5).

Figure 4: Percent of respondents who suspected a corn rootworm trait failure in at least one of their cornfields in 2012.

If you find injury to Bt corn on more than one node, the ISU entomologists say you should consider diversifying pest management for that field. Severe injury may be due to western corn rootworm that has developed Bt resistance, and resistance should be suspected in these cases. The Department of Entomology at Iowa State University is working to understand the extent of resistance within the state and to develop management recommendations for Bt-resistant populations of western corn rootworm. Contact Aaron Gassmann or Erin Hodgson to report fields with suspected resistance.

Erin Hodgson is an associate professor of entomology with Extension and research responsibilities; contact her at [email protected] or 515-294-2847. Kristine Schaefer is program manager of the pest management and the environment program and can be reached at [email protected] or 515-294-4286. Aaron Gassmann is an assistant professor of entomology with research and teaching responsibilities in insect pest management; contact him at [email protected] or 515-294-7623.